The Mighty Archetype: The Designated Psychopath

Totally relevant to this post: you guys have to watch this amaaaazing AMV (anime music video) of Soul Eater (which you should TOTALLY watch and read) set to This is Halloween sung by Marilyn Manson:

The Designated Psychopath - This is a fun archetype because the things you can do with this character are practically limitless. Logic and rationale is absent because they are, quite literally, insane.

Gaara grew up with a powerful demon sealed inside him. This made him a target of hatred and fear which, in combination with the demon that could influence him, made him mentally unstable, which resulted in a rather murderous personality. Before the events of the manga, Gaara had never before been physically injured because the sand in his gourd protects him of its own will. He calls it his Perfect Defense.
He can also manipulate the sand to do whatever he wants (he can even ride it like a giant sand surfer), and his signature move involves wrapping his enemies in his sand and then, simply by closing his fist, crushing them.

Crona was raised since birth to kill, even though she never wanted to. Her mother is a witch (literally) who often starved her until she agreed to kill small animals. This was her childhood:

I don't know about you, but my heart breaks for these sorts of characters. You can always bet that there's some sort of horrific trauma behind the insanity. Well, usually. A character who's psychotic simply because can be fun, too, but I prefer the ones who are a bit more complicated.

I mean, look at them. It's no wonder the main characters always want to help redeem them:

Once Gaara met Naruto, who had also grown up the object of hatred because of a demon sealed inside him, Gaara learned that hatred and solitude weren't his only options. He went on to become the leader of his village, its protector instead of its destroyer. (I love, love, love Gaara) Crona began as an enemy as well. Then there was a brief time in which she and the MC became amazing friends, and Crona seemed happy. But her mother returned and twisted her fragile mind again.

The thing about these characters is that you can push and push and push and literally see how their minds break--and it's tragic and sad and horrifying. Sometimes they can be redeemed and brought back from the edge... and sometimes they can't. Either way, working out how to do it is incredibly interesting.

Finding ways to test these characters can often be a great way to test your main characters as well!